Known in the past has been an exhaust purification system which provides with an air-fuel ratio sensor at an upstream side, in the exhaust flow direction, of an exhaust purification catalyst provided in an exhaust passage of the internal combustion engine, and provides with an oxygen sensor at a downstream side of the exhaust purification catalyst in the exhaust flow direction. In such an exhaust purification system, for example, the amount of fuel fed to the internal combustion engine is controlled by feedback based on the output of the upstream side air-fuel ratio sensor such that the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst becomes a target air-fuel ratio (main feedback control) and the target air-fuel ratio is controlled by feedback based on the output of the downstream side oxygen, sensor (sub feedback control).
In the above-mentioned feedback control, the output values of the upstream side air-fuel ratio sensor and downstream side oxygen sensor are utilized. For this reason, if abnormalities in these air-fuel ratio sensor and oxygen sensor, cause large error to occur in their output values, feedback control becomes unable to be suitably performed. For this reason, an abnormality diagnosis system which diagnoses an upstream side air fuel ratio sensor and a downstream side oxygen sensor for abnormality has been proposed (for example, PLT 1).
For example, in the abnormality diagnosis system described in PLT 1, during operation of the internal combustion engine, abnormality of the oxygen sensor is diagnosed based on the response time from when starting fuel cut control which stops feed of fuel to the internal combustion engine to when the downstream side oxygen sensor changes in output value. In particular, when this response time is an abnormality judgment value or more, it is considered that the oxygen sensor has fallen in responsivity and it is judged that the oxygen sensor has become abnormal.
On the other hand, an exhaust purification catalyst also deteriorates the longer that it is used. If the exhaust purification catalyst deteriorates in this way, it is known that along with this, the exhaust purification catalyst decreases in maximum storable oxygen amount. For this reason, by detecting the maximum storable oxygen amount of the exhaust purification catalyst, it is possible to detect the degree of deterioration of the exhaust purification catalyst. As the method of detection of this maximum storable oxygen amount, for example, active air-fuel ratio control in which the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst is alternately switched between the rich air-fuel ratio and the lean air-fuel ratio is known. In this method, the maximum storable oxygen amount of the exhaust purification catalyst is estimated based on the output of the downstream side oxygen sensor, which changes along with active air-fuel ratio control (for example, PLT 2).